The present invention relates generally to digital sound synthesizing devices which synthesize a sound signal by generating an oscillating signal by use of a closed wave guide network, and more particularly to a digital sound synthesizing device which modifies the pitch of sound to be synthesized in consideration of signal delay time given by a filter provided in the closed loop of a wave guide network.
U.S. Pat. No. 5,212,334 discloses the fundamental structure for generating and synthesizing a tone waveform signal by use of a closed digital wave guide network. According to the disclosed technique, delay circuitry, filters etc. are connected in closed loop to form a signal circulating path. A digital exciting signal is introduced into and circulated in this circulating path to thereby generate a waveform signal, and then an output tone waveform signal is taken out from a suitable point in the loop. This technique is essentially based on the concept of modelling the physical characteristics of a desired natural musical instrument such as a wind or stringed instrument by means of a closed digital wave guide network to thereby simulate a tone of the natural musical instrument. That is, the signal circulating path (closed digital wave guide network) models the physical propagation of an oscillating signal progressing or reflecting within a medium such as a tube or string of the musical instrument. In the case of simulation of the wind instrument, the above-mentioned signal circulating path corresponds to the tubular portion of the instrument, and the signal delay time simulates the length of the tube to thereby set the resonance characteristic of the tubular portion. Further, the filters inserted in the signal circulating path simulate attenuation and other frequency characteristics of sound waves at the end portion, opening, aperture etc. of the tube, so as to control the color or timbre of a tone to be generated. On the other hand, in the case of simulation of the stringed instrument, the above-mentioned signal circulating path corresponds to the string portion of the instrument. As mentioned, the signal circulating path corresponds to the oscillation generating section of the physical tone source.
The signal circulating path is provided with signal junction sections, as may be necessary, which model the progression and reflection of signals in physical boundaries (oscillation exciting section such as a reed, aperture formed in the tube, mounted ends of the string etc.) along the propagation path of the oscillating signal. For example, the signal junction section for modelling the oscillation exciting portion includes a non-linear conversion section. The signal junction section for modelling the other physical boundaries includes arithmetic operation circuits for separating and synthesizing progressing and reflecting wave signals. The above-mentioned non-linear conversion section for exciting the oscillating signal includes for example a non-linear table. By introducing into the loop of the signal circulating path a suitable electrical pressure signal corresponding to breadth pressure or string-scraping operation, a signal resultant from non-linearly converting the pressure signal by use of the non-linear conversion table is caused to circulate in the signal circulating path so that an oscillating waveform signal is excited. In another case, a noise signal or suitable initial waveform signal is used as the signal to be introduced into the signal circulating path for exciting an oscillating waveform signal.
The signal delay time in the signal circulating path can be variably controlled by changing the number of delay stages in the delay circuit provided within the signal circulating path, so that it is allowed to control the resonance characteristic in the circulating path and thus set/control the pitch of a tone waveform signal to be synthetically formed in the circulating path. In this case, the unit delay time (minimum unit delay time, i.e., delay time given by one delay stage, namely, one delay clock or one sampling clock time) in the delay circuit is constant.
Because each of the filters provided within the signal circulating path presents a phase delay characteristic as well as its original amplitude-frequency characteristic, a very slight signal delay corresponding to the phase delay would undesirably occur in the signal circulating path. Such signal delay time caused by the filter would vary depending on the filter coefficient (i.e., cut-off frequency) and on the signal frequency as well. Particularly, such a phase delay characteristic is very appreciable in an IIR (infinite impulse response) filter having a feedback loop within a filter circuit. Accordingly, the total delay time in the signal circulating path equals the sum of the delay time set to the delay circuit and the signal delay time provided by the filter (if any other delay element is present in the closed loop, the delay time provided by the other delay element as well should, of course, be considered). Thus, in order to synthetically form a tone signal of desired pitch, it is not sufficient to only set the delay time of the delay circuit in accordance with the desired pitch, and it is necessary to compensate for the additional signal delay time provided by the filter.
In this type of technique, the fundamental pitch adjustment method is by changing the number of delay stages in the delay circuit, but changing the number of delay stages alone can only achieve adjustment to an extent corresponding to the unit delay time. In order to compensate for the signal delay time provided by the filter, it is necessary to perform minute adjustment to an extent smaller than the unit delay time in the delay circuit, and hence the intended compensation requires some special approach. Among the traditional pitch adjustment methods is known a technique of interpolating between output signals from different delay stages of the delay circuit (namely, "inter-stage interpolation"). In the past, such an inter-stage interpolation technique was solely employed for achieving a pitch modulation effect such as vibrato, and the use of the inter-stage interpolation for compensation for the signal delay time introduced by the filter was not proposed or considered at all. What should be given particular attention in connection with this type of inter-stage interpolation technique is that an interpolation circuit inserted in the closed loop of the signal circulating path would undesirably function as a low-pass filter to thereby attenuate the high-frequency components of a tone signal more than necessary. Further consideration should be given so as not to cause noise etc. due to delay control operations performed for the required compensation, in attempting to achieve a time-variation in tone color characteristic by time-varying the filter coefficient during generation of the tone.
U.S. Pat. No. 5,308,918 discloses such an inter-stage interpolation technique. Japanese Patent laid-open Publication No. HEI 2-267594 discloses a technique of controlling the delay amount of a delay circuit in response to a change in a filter coefficient, and Japanese Patent laid-open Publication No. HEI 6-67674 discloses a technique of controlling the delay in a closed loop by use of an all-pass filter.